Question

根据python文档，x<y<z比较转换为x<y and y<z，表达式y最多只评估一次。
现在我的问题是，表达式y（看下面的代码）在这里只评估一次吗？

if(x<y and y<z):

Answer 1

两次：

>>> def f():
...   print "F called"
...   return 1
...
>>> 0 < f() < 100
F called
True
>>> 0 < f() and f() < 100
F called
F called
True
>>> if (0 < f() and f() < 100):
...   print True
...
F called
F called
True
>>>

Answer 2

没有

>>> dis.dis(lambda x, y, z: x < y() < z)
  1           0 LOAD_FAST                0 (x)
              3 LOAD_FAST                1 (y)
              6 CALL_FUNCTION            0
              9 DUP_TOP             
             10 ROT_THREE           
             11 COMPARE_OP               0 (<)
             14 JUMP_IF_FALSE            8 (to 25)
             17 POP_TOP             
             18 LOAD_FAST                2 (z)
             21 COMPARE_OP               0 (<)
             24 RETURN_VALUE        
        >>   25 ROT_TWO             
             26 POP_TOP             
             27 RETURN_VALUE        
>>> dis.dis(lambda x, y, z: x < y() and y() < z)
  1           0 LOAD_FAST                0 (x)
              3 LOAD_FAST                1 (y)
              6 CALL_FUNCTION            0
              9 COMPARE_OP               0 (<)
             12 JUMP_IF_FALSE           13 (to 28)
             15 POP_TOP             
             16 LOAD_FAST                1 (y)
             19 CALL_FUNCTION            0
             22 LOAD_FAST                2 (z)
             25 COMPARE_OP               0 (<)
        >>   28 RETURN_VALUE

Python比较评估

2 个答案: